Recent trends of increase in size of structures and weight saving of automotive parts have been raising a need for high-strength metal materials stronger and tougher than ever. Among these, steel materials, such as sheet steel and shaped steel, have been demanded to have high strength. At the same time, there has been also demanded strengthening of bolt that is used for jointing those steel materials (see, for instance, Patent Document 1 and Non-Patent Document 1).
Mechanical properties required for materials of the bolt are: (1) good formability; (2) high resistance to delayed fracture; (3) excellent resistance to environmental deterioration; (4) excellent impact toughness; and the like. However, these properties and an increase in strength of materials are in a trade-off relationship.
In the steel material having a tensile strength of more than 1,200 MPa, especially a delayed fracture is a serious problem, which hinders strengthening of the high-strength bolt. The delayed fracture, a shortened term for “time-delayed fracture”, is a fracture resulting from embrittlement of the steel material caused by hydrogen which has been generated by an atmospheric corrosion and intruded into steel material. The cause of the delayed fracture is thus the hydrogen which is diffused and concentrated in the steel at room temperature. Because of this delayed fracture, development of the high-strength bolt for construction works had been stagnated at a tensile strength of up to 1,100 MPa for approximately 30 years until late 1990s when a super-high-strength bolt having a tensile strength of 1,400 MPa was developed (see, for example, Non-Patent Document 1).
In general, the production of high-strength bolt includes the following steps: softening a steel material; forming a bolt head portion by cold heading; forming a threaded portion by cold rolling; and quenching and tempering of the bolt. Patent Document 2 discloses that the high-strength steel for machine structure having a tensile strength of 1,800 MPa or more and excellent delayed fracture resistance was obtained by defining tempering conditions and additive amounts of C, Si, Mn, Cr, and Mo. Also, a production method and mechanical properties of the super-high-strength bolt using this 1,800 MPa-class high-strength steel for machine structure has been reported (Patent Document 2). However, it is pointed out that, with respect to the above high-strength steel material, it is difficult to soften the material and form a head portion by cold heading, and in the case of bolt shape be defined in accordance with Japanese Industrial Standard (JIS), the delayed fracture property has not yet been completely overcome (Non-Patent Document 2).
Since the quenching and tempering process of bolt is complicated, there is a production method in which the quenching and tempering treatment is omitted (untempered bolt). Patent Document 3 discloses that a wire rod that was prepared by heavily cold drawing of steel material having fine pearlite structure was used as a blank to obtain a bolt-shaped material by cold heading and that the delayed fracture property and relaxation can be ameliorated by performing a strain ageing treatment to the bolt-shaped material. Patent Document 1 also discloses that, after forming a bolt, low-temperature toughness was improved by applying a tensile stress of an elastic limit or lower to the bolt and performing a heat treatment thereto. However, since these methods are performed on condition that the bolt is formed in cold working, a shape and a size of the bolt are limited. Further, since it is necessary to set an amount of carbon to 0.7 percent by weight or more, a remarkable improvement of impact toughness cannot be expected. Herein, percent by weight is equivalent to percent by mass.